The present invention is concerned generally with sealing glass compositions in powdered form for sealing glass components such as television picture tubes, and particularly with a sealing glass composition that forms excellent devitrified seals when processed at a low temperature or for a short time compared to conventional sealing glasses.
Crystallizable sealing glass compositions in powdered form have been used to provide devitrified glass seals between the panels and funnels of television picture tubes. The resultant seals must have a suitable combination of properties such as appropriate thermal expansion characteristics to avoid damage to the picture tube components, good flow to produce proper fillet shape, good wetting to provide strong adhesive characteristics to the glass parts being sealed, and good crystallization properties to allow formation of a strong crystallized seal within a reasonable thermal soak time.
Properties important to the formation of a strong seal include the rate of crystallization and the size and distribution of crystals. The ideal crystallization rate is fast but delayed enough to allow adequate chemical bonding, sometimes referred to as "wetting," of the prepared sealing glass to the panel and funnel glasses. The formation of large, fast-growing crystals within the glassy matrix results in an intrinsically strong seal. Crystals that tend to form an interlocking structure rather than flower-like forms are preferred because they have provide greater resistance to crack propagation.
U.S. Letters Patent No. 4,589,899 to Hudecek, incorporated herein by reference, discloses a crystallizable PbO/ZnO/B.sub.2 O/SiO.sub.2 /BaO glass sealing composition in powdered form that includes finely divided zinc zirconium silicate as a nucleating agent. This composition has a wide sealing temperature tolerance (sealing at 440.degree. C. to 460.degree. C. in about 40 minutes) and a satisfactory balance of physical properties necessary to produce an acceptable seal, including good wetting characteristics and high intrinsic seal strength. The zinc zirconium silicate nucleating agent is thought to be a factor in achieving these properties in the Hudecek glass.
The processing temperature for sealing glasses depends, among other things, on the design of the furnace and the size of the tube to which the sealing glass has been applied. Notwithstanding these variables, conventional sealing glasses generally, and the Hudecek glass particularly, must be processed at temperatures of about 440.degree. C. to 460.degree. C. for about 40 minutes to achieve satisfactory sealing properties. The Hudecek composition, for example, does not have satisfactory sealing properties when it is processed at lower temperatures (e.g., in the range of about 420.degree. C. to about 435.degree. C.) or for shorter times (e.,g., for about 15 to about 20 minutes).
The high processing temperatures and long processing times of such conventional sealing glasses use more energy than would be required for a sealing glass capable of being processed at lower temperatures and/or for shorter times. More importantly, the long processing time severely limits the potential production in a processing facility having a fixed capacity. Shorter processing (soak) times would permit the sealing of more picture tube components in any given interval without the need for expansion of the processing facilities. Lower processing temperatures also may decrease ramp-up and ramp-down processing time to the extent that shorter heating approach times (usually in the range of about 3.degree. C. to about 6.degree. C. per minute from about 375.degree. C. to about 440.degree. C.) and shorter cooling times (usually in the range of about 2.5.degree. C. to about 7.degree. C. per minute to prevent breakage) are required. Energy costs are or will be reduced with lower processing temperatures and shorter processing times.
A need exists for a crystallizable sealing glass in powdered form for use in sealing television bulbs which can be processed at faster rates and at lower soak temperatures without a loss of sealing characteristics, and particularly wetting and seal strength characteristics.
One object of the present invention is to provide a crystallizable glass sealing composition in powdered form which has superior processing characteristics compared to conventional sealing glasses, namely, the ability to be processed at a lower temperature, e.g., in the range of about 420.degree. C. to about 435.degree. C., rather than at about 440.degree. C., or for a shorter time, e.g., for about 15 to about 30 minutes, rather than for about 40 minutes, or both.
A second object of the present invention is to provide a glass sealing composition in powdered form comprising a crystallizable PbO/ZnO/B.sub.2 O.sub.3 /SiO.sub.2 glass and finely divided zinc zirconium silicate. The glass has a lower sealing time than conventional sealing glasses, forming a complete seal in about 15 minutes at a hold temperature range of about 420.degree. C. to about 435.degree. C. rather than about 40 minutes at about 440.degree. C.
A third object of the present invention is to provide a sealing glass composition in powdered form comprising a crystallizable PbO/ZnO/B.sub.2 O.sub.3 /SiO.sub.2 glass in the vitreous state and an effective amount of a nucleating agent. The glass composition of the present invention provides an excellent devitrified glass seal with a lower temperature thermal soak than conventional glass compositions.
Another object of the present invention is to provide a method of sealing a crystallizable glass sealing composition in a television picture tube component, the method including:
A. providing a crystallizable glass having the following ingredients in approximate percent by weight: PA1 B. mixing finely divided particles of the glass with an effective amount of a nucleating agent, preferably zinc zirconium silicate with an average particle size of about 2 to about 8 microns, and; PA1 C. heating the glass composition to a hold range of about 420.degree. C. to about 435.degree. C. to provide an excellent devitrified seal.
Ingredient Percent By Weight PbO 79 ZnO 10.4 B.sub.2 O.sub.3 8.4 SiO.sub.2 2.2 BaO 0-1.5
These and other objects of the present invention will be apparent from the specification that follows, the appended claims, and the drawings.